1. Field of the Invention
The present invention relates to a process for manufacturing a reflection-type liquid crystal display apparatus which can be utilized in office automation equipment such as personal computers and the like.
2. Description of the Related Art
In recent years, since office automation equipments such as personal computers and the like have become more and more portable, the reduction in cost of their display apparatus has become an important issue. The display apparatus comprises a pair of oppositely arranged substrates on which electrodes are respectively provided which sandwich a display medium having electrooptical characteristics. A display is generated by applying a voltage to both electrodes. The display media that can be used includes liquid crystals, electroluminesense, plasma, electrochromic, and the like. Among these media, liquid crystal display (LCD) apparatuses are the most common since they are capable of generating a display which consumes a lower amount of power.
Regarding the display mode and driving method of the liquid crystal display apparatus, a simple matrix type including a super twisted nematic (STN) is a class which can most realize the cost reduction. However, because there are demands for display apparatus having higher resolution, higher contrast, multi-gradation (multi-color, full color), wide viewing angle characteristics and the like with information being transferred through multimedia, it would be difficult in the future to fulfill these demands by using a simple matrix type.
An active matrix type has been proposed which comprises a switching element (an active element) on each pixel to increase the number of scanning lines, and according to this type, the display apparatus having improved resolution, contrast, multi-gradation, wide viewing angle characteristics and the like can be obtained. According to this active-matrix type liquid crystal display apparatus, pixel electrodes are provided in a matrix shape on one of a pair of oppositely arranged substrates which sandwich a liquid crystal layer, i.e., an active matrix substrate, while scanning lines are provided passing through the vicinity of the pixel electrodes, both of which are electrically connected via an active element.
The active element is a two-terminal nonlinear element or a three-terminal nonlinear element. Representative active elements that have presently been employed include thin film transistors (TFT) which are three-terminal nonlinear elements.
Furthermore, in recent years, since a demand for lowering the consumed power is increasing, a reflection-type liquid crystal display apparatus has been eagerly developed instead of a transmission-type liquid crystal display apparatus which usually requires a backlight.
It is necessary to increase the strength of a light scattering toward a direction vertical to the display screen with respect to incoming light at any angle in order to achieve a bright display in this reflection-type liquid crystal display apparatus. Thus, it is necessary to manufacture a reflector having optimum reflection characteristics. A process for manufacturing a reflector comprising forming a controlled uneven surface of a substrate made from for example glass, etc., and forming thereon a thin film made from silver, etc., is known in the art. For example, Japanese Laid-open Publication No. 6-75238 (Nakamura et al.) describes forming a plurality of convex portions by applying a photosensitive resin on a substrate, and by exposing and developing the photosensitive resin through a light-shading means having a circle light-shading region arranged thereon. In this publication, an insulating protective film is then formed on the convex portions along with their shape, and a reflector consisting of a metallic thin film is formed on the insulating protective film.
When the reflector is formed outside of the substrate (i.e., the side opposite to the liquid crystal layer), a double image occurs due to the effect of the thickness of the glass substrate, but this problem may be overcome by forming the reflector inside of the glass substrate (i.e., the liquid crystal layer side) so as to use the reflector as a pixel electrode also.
The following illustrates conventional reflection-type liquid crystal display apparatus with reference to the drawings.
FIG. 11 is a plan view showing the structure of an active matrix substrate in conventional reflection-type liquid crystal display apparatus, FIG. 12 is a plan view showing the structure of one pixel portion in the active matrix substrate; FIG. 13A is a plan view showing the structure of a gate terminal portion in the terminal region located outside of the display region (the screen portion) on which a pixel electrode is formed in the active matrix substrate; FIG. 13B is a plan view showing the structure of a source terminal portion; FIGS. 14Aa to 14Fa are sectional views illustrating a process for manufacturing the C-C' line sectional portion, i.e., the TFT portion of FIG. 12; FIGS. 14Ab to 14Fb are sectional views illustrating a process for manufacturing the D-D' line sectional portion, i.e., the gate terminal portion of FIG. 13A; and FIGS. 14Ac to 14Fc are sectional views illustrating a process for manufacturing the E-E' line sectional portion, i.e., the source terminal portion of FIG. 13B.
As shown in FIG. 12, the active matrix substrate comprises a pixel electrode (a reflection electrode) 505 which is formed in a matrix shape on a glass substrate 501 and also used as a reflector, and a gate bus line 502 as a scanning line and a source bus line 503 as a signal line both of which are formed crossing each other while passing through the vicinity of the pixel electrode. A TFT 504 as a switching element is provided in the vicinity of the crossing portion between each gate bus line 502 and each source bus line 503. A gate electrode of the TFT 504 is a portion branching from the gate bus line 502, while a source electrode of the TFT 504 is a portion branching from the source bus line 503, and a drain electrode of the TFT 504 is electrically connected to the reflection electrode 505.
In the TFT portion, a gate electrode 502a is formed on the glass substrate 501, and an insulating layer 506 is formed as covering its upper portion, as shown in FIG. 14Fa. A semiconductor layer 507 is formed thereon as overlapping the gate electrode 502, and contact layers 520a and 520b are formed as covering a portion of the semiconductor layer 502a and being separated from each other by a center portion. A source electrode 503a is formed on one contact layer 520a, while a drain electrode 508 is formed on the other contact layer 520b. A reflection electrode 505 made of Al or the like, is formed partially overlapping the drain electrode 508 of the TFT 504, and electrically connected to the drain electrode 508.
In the gate terminal portion, a connector electrode 509, connected to outside elements, is formed on the gate bus line 502 which is formed on the glass substrate 501, as shown in FIGS. 13A and 14Fb. Also, in the source terminal portion, a connector electrode 509, connected to outside elements, is formed on the source bus line 503 which is formed on the glass substrate 501, as shown in FIGS. 13B and 14Fc.
The active matrix substrate can be manufactured for example as described below in the reflection-type liquid crystal display apparatus which also uses the pixel electrode as a reflector. Since the TFT portion can be manufactured in the same manner as that of conventional liquid crystal display apparatus until a TFT 504 as shown in FIG. 14Aa is formed, and the gate terminal portion and the source terminal portion can also be manufactured in the same manner as that of conventional liquid crystal display apparatus until a connector electrode 509 made of ITO (indium tin oxide), as shown in FIGS. 14Ab and 14Ac, is formed on the gate bus line 502 and the source bus line 503, the following illustrates a process for manufacturing them after these steps.
First, a reflection electrode film 505' is formed on the entire surface of a substrate, as shown in FIGS. 14Ba, 14Bb and 14Bc.
Then, a resist is applied on the reflection electrode film 505' to form a resist film 510, as shown in FIGS. 14Ca, 14Cb and 14Cc.
Subsequently, the resist film 510 is exposed and developed using a photolithography technique to form a resist pattern 510' for the formation of a reflection electrode in the TFT portion, as shown in FIG. 14Da. At this time, the resist film 510 in the gate terminal portion and the source terminal portion is removed, as shown in FIGS. 14Db and 14Dc.
Thereafter, the reflection electrode film 505' is etched using the resist pattern 510' as a mask to form a reflection electrode 505 in the TFT portion, as shown in FIG. 14Ea. At this time, the reflection electrode film 505' in the gate terminal portion and the source terminal portion is removed, as shown in FIGS. 14Eb and 14Ec.
Then, the substrate is for example immersed into a resist peeling solution to remove the resist pattern 510'.
As described above, the reflection electrode 505 connected to the drain electrode 508 is revealed in the TFT portion as shown in FIG. 14Fa, while the connector electrode 509 formed on the gate bus line 502 and the source bus line 503 is revealed in the gate terminal portion and the source terminal portion as shown in FIGS. 14Fb and 14Fc.
In such a reflection-type liquid crystal display apparatus, the reflection characteristics of the reflection electrode 505 may be optimized by forming a plurality of convex portions and a plurality of concave portions in a portion near which the reflection electrode 505 is formed in order to increase the strength of a light scattering toward a direction vertical to the display screen with respect to incoming light at any angle.
In this case, these convex portions and concave portions may be formed as for example shown in FIGS. 15A to 15D. First, a photosensitive resin 511 is applied onto the surface of a glass substrate 501 which constitutes an active matrix substrate, as shown in FIG. 15A. Then, the photosensitive resin 511 is exposed through a light-shading means 514 (reticle) having a light-shading region 512 and a light-transmitting region 513, as shown in FIG. 15B and thereafter developed to form a photosensitive resin pattern 511', as shown in FIG. 15C. Thereafter, the photosensitive resin pattern 511' is subjected to a heat treatment to form unevennesses 511" having optimum shapes, as shown in FIG. 15D.
The light-shading means 514 that can be used include, for example, those having circular light-shading regions arranged therein as shown in FIG. 16, and those having circular light-transmitting regions arranged therein as shown in FIG. 17.
According to such a conventional reflection-type liquid crystal display apparatus, the reflection electrode film 505' is etched to form the reflection electrode 505 as shown in FIG. 14Da in a wet etching procedure using a nitric acid/acetic acid/phosphoric acid/water system as an etchant.
Apparently, it is preferred that a material having a higher reflectivity be used for the reflection electrode. For that reason, Ag is an optimum reflection electrode material, but Ag has a higher diffusion rate and thus may cause problems when Ag diffuses into and reacts with a base substrate.
On the other hand, because Al, which is less likely to diffuse into and react with a base substrate, has been widely used for the metallization of integrated circuits with good characteristics such as etching conditions and the like, Al is often used as a material for the reflection electrode.
In this case, a reflection electrode film 505' made of Al is formed and etched on a connector electrode 509 made of ITO in the gate terminal portion and the source terminal portion, as shown in FIGS. 14Db and 14Dc.
Since a thin film has an extraordinary number of lattice defects compared to a bulk-state material and includes imperfect crystals, there are a number of pinholes produced in the reflection electrode film.
For example, as shown in FIG. 18 which is an enlarged sectional view of the circled portions in FIGS. 14Cb and 14Cc, a number of pinholes are produced in the reflection electrode film 505' formed on the connector electrode 509 made of ITO which is located on the gate bus line 502 and the source bus line 503.
When the resist film 510 is exposed and developed under the above-described condition, the developer would be in contact with the reflection electrode film 505' as well as the connector electrode 509 through these pinholes 515.
In a case where the reflection electrode film 505' is made of Al and the connector electrode 509 is made of ITO as described above, the developer would be interposed between the reflection electrode and the connector electrode to provide a cell effect, which leads to a reaction between Al and ITO causing corrosion and dissolution between them. This would certainly reduce the yield of the TFT, i.e., the reflection-type liquid crystal display apparatus.
As described above, there is a demand for a process for manufacturing a reflection-type liquid crystal display apparatus which is capable of preventing the corrosion and dissolution caused by a cell effect between the connector electrode and the reflection electrode film at the time of patterning the reflection electrode film, and thus improving the yield thereof.